This invention relates to sheet feeding devices suitable for use with optical character readout apparatus, printers, copy machines, etc., and more particularly, to a sheet feeding device capable of stably carrying out separation and feeding of sheets of less than 55 kg paper.
In this specification, the term "55 kg paper" refers to sheet having a characteristic such that, if the sheets have a size 788 mm.times.1091 mm, the sheets have a weight of 55 kgf in 1,000 sheets.
Recently, there has been a demand to carry out rationalization of office work and various kinds of office automation equipment have been developed. The majority of office work is accounted for by paper work consisting of making and filing documents. To rationalize such work, it is important that input devices for reading the information recorded on a paper and output devices for printing out the results of calculation have their performance improved. For example, optical character read-out apparatus and various printers have important functions as input and output devices for office work. Meanwhile, in this type of work, accumulation and transfer of information relies on sheets as a medium in many cases, and in practice the volume of sheets used in office work is increasing. Consequently, to use sheets of a small thickness for office work is an important requirement for conserving natural resources and reducing office space. However, automatic sheet feeding devices of the prior art are only able to handle sheets of a large thickness such as sheets of over 55 kg paper. When the sheets used are less thick, the rigidity of the sheets is reduced and difficulties are experienced in handling the sheets, resulting in double feeding or sheet jamming. Thus, the aim of achieving rationalization of office work is defeated.
For example, an optical character read-out apparatus can generally only handle sheets of relatively high thickness and rigidity which are of 70-135 kg paper.
Presently, there are two types of practical processes for individually separating a sheet from a stack of sheets stored in a hopper and feeding the separated sheets. One proposed process relies friction when feeding thin sheets, the following problems arise.
To attract a sheet by a vacuum pump, thin sheets are air-permeable and not only one sheet but two or more sheets are attracted by the force of vacuum, thereby causing double feeding to occur. A process is available which relies on subatmospheric pressure in attracting sheets for separating one sheet from the rest of the sheets. However, this process suffers a disadvantage in that a large capacity blower is required and the apparatus for working the process is relatively large. Additionally, the blower generates considerable noise, so that it is not possible to reduce the size and noise level.
Meanwhile a frictional separation mechanism used in many copying apparatus, printers, etc., also have the problems of sheet jamming, sheet bending and wrinkle formation due to a lack of rigidity in the processed sheets.
An object of the invention is to provide a sheet feeding device of high reliability capable of avoiding buckling or jamming of thin sheets of for example, less 55 kg in ream weight, when being fed to a subsequent processing station.
Another object of the invention is to provide a sheet feeding device capable of avoiding a skew movement of the thin sheet.
A sheet feeding device according to the present invention comprises feeding means for exerting a feeding force P (gf) on the uppermost sheet and separating means for offering resistance to the sheets fed by the feeding means. A distance L (mm), in the feeding direction, between a point at which the feeding means exerts the feeding force on the sheets and a point at which the separating means exerts a separating force on the sheets is set in a range defined by the following formula so that no buckling of the thin sheets is produced: ##EQU2##
In another aspect of the invention, the feeding means comprises a plurality of feeding members separated from each other in a direction perpendicular to the feeding direction so as to avoid bending and a skewing movement of the thin sheets.